Casting of the Year: Planets Aligned for Clean-Sheet Cadillac Casting Design

Kim Phelan

By the end of 2023, Cadillac will make its first customer deliveries of the highly-customizable, all-electric, and ultra-luxury Celestiq sedan, GM’s most expensive haute-design car ever, starting at $300,000. But major cost-reducing castings are expected to be a key spigot on profit for the high-profile and highly-anticipated rollout.   

The vehicle’s marketing––including its name––evokes otherworld futurism; one color scheme is described as “an ode to infinity inspired by the mystery of the cosmos.” The curious copywriting aside, that which is unseen in Celestiq’s structure is equally sophisticated as its visible, iconic features; and for one remarkable underbody casting, one might even say the planets were aligned as collaborative engineering, proprietary material and process, and state-of-the-art technology produced an award-winning synergy.

Celestiq’s rear rail underbody casting, manufactured by AFS Corporate Member Tooling & Equipment International (TEI) in Livonia, Michigan, has been named AFS Casting of the Year. Two rear rails are among a total of six giant castings, which, when connected, make up the car’s entire underbody, essentially the whole structure—and the most critical part—of the vehicle, according to TEI President Oliver Johnson. Quite literally, there’s a lot riding on the underbody, to which hundreds of components are attached, including the suspension, battery, and the full body. 

A completely clean-sheet design with no predecessor, the 59.5-lb. casting measures in at 58.5 x 22.5 x. 33.3 inches. It was made with TEI’s own proprietary sand cast alloy developed for high ductility (bendability), an essential property for the automotive industry. Produced with low-pressure precision sand casting, the rear rail combines the rail, shock tower, and wheel well into a single casting, thereby replacing many stamped components in a traditional design. Cored box sections give it enormous strength and stiffness, Johnson said, and all internal cores are produced using additive manufacturing. Employing 3D-printing for the cores opened up a world of options for Cadillac’s underbody engineering team, led by Engineering Group Manager Ed Moss—options and geometry that would not be producible with conventional tooling. 

“By using the 3D-printed cores, we were able to introduce all these internal features—bosses and webs,” Johnson said. “Really, they had complete design freedom to do whatever they wanted inside the casting, even though you can’t see it.”

Exterior and interior expressive design was a high priority for the Celestiq program, but design features like a low roof and big tires take geometry away from the structure, said Moss at GM. However, he and his team were able to make up for geometry losses and regain structure with the ingenuity of 3D-printed cores. Their patent-pending casting design includes what he calls a rail in a rail—inner ribbings that form an intermediate rail inside the casting to carry loads.

“With TEI’s process and their flexibility with 3D core printing, the internal geometry is amazing,” Moss said. “We could actually make a part with the exact geometry, the exact properties as what we’re going to do in production by 3D printing the molds, and actually it was a very successful test. It showed us that we’re on the right course; no surprises. That was really kind of a stepping stone for product assurance so we felt comfortable. Because again, we’re doing this for the first time—it’s the first time we’ve done this type of vehicle with these types of castings.”

Unique Process

TEI recommended 3D printing during the collaborative process between the foundry and Moss and his team. The idea at first created division among the carmaker’s body group, some members uncomfortable with the higher cost associated with 3D printed cores. In the end, the flexibility it afforded with internal geometry won out. The customer also gained the priority properties of (A) strength from box sections and (B) crashworthiness due to internal details that transfer forces inside the boxes.

The foundry owns three VX4000 voxeljet sand printers that print 4 meters x 2 meters in one build—totaling 63 ft. in length, each with two tables that move in and out. They’re about the size of a small house, Johnson said. The industrial scale of the equipment is highly compatible with the low-volume, high-spec Cadillac project, he added. To its state-of-the-art set-up TEI adds a proprietary combination of process and product, which they developed and fine-tuned over the last decade.

“At TEI we use the additive manufactured cores to create superior castings in two ways,” said Johnson. “One is that we actually use them within our gating system. So even though we build tooling to make these castings, because we’re making hundreds of them every year, we still use 3D-printed cores, because, by putting them into our tooling and then over molding them with airset sand, we can change the direction of the flow of the metal within the mold but with a relatively simple tool. So additive manufacturing is used in our process to alter the way the metal flows into the casting and enables us to feed parts of it that are hard to access. 

“Second, is using the 3D printing for all these internal box sections and the geometry we’ve talked about,” he added. “So there’s really two ways in which we’re using these additively manufactured cores, and I’m not aware of any other company ever doing this for the feed paths and the gating.”

The result is a nearly homogenous part, said Moss, who noted that what TEI does with low-pressure sand casting actually mimics high-pressure diecasting. 
“Throughout this very large part, with their unique process, you’re able to get consistent properties, which is very rare in casting,” Moss said. “Now, you could not do this with 100,000 units—this is a prototype process that we parlayed into low volume production. Being a low-volume car, we wanted to take every advantage we could with precision sand casting.”

Another benefit of TEI’s processes was great accuracy of the castings—TEI produced four others besides the two rear rails to complete the full underbody. Johnson said when Moss’s team put the first castings together, they were amazed at how precise they were, all interfaces between the six big castings having been machined. 

“They’re all very accurate compared with when you’re taking a bunch of stampings and welding them together,” Johnson said. “So, they’re able to achieve much, much higher quality in terms of dimensional accuracy and much quicker than they were used to.”

Proprietary Alloy

TEI brought another novel solution to the table, recommending its own proprietary alloy for the rear rail casting. Not certified by any standards/specifications board, the alloy has been intentionally kept under wraps, which meant the GM team would perform hundreds of sample tests to assure itself on necessary properties. 

Moss said the alloy, known as TTA (TEI tactical aluminum), gave the parts both high elongation and higher yields, allowing his team further optimization than they’d expected at the onset. 

Johnson explained TTA is a low-silicone variant of a 300 series alloy that has very tightly controlled levels of magnesium and other elements. 
“We developed it specifically for high ductility applications, but not just ductility in tension,” he said. “When you talk to metallurgists, normally you’re talking about measuring elongation, ultimate tensile strength, and yield. But really what’s important in a crash is not just tension; it’s crumpling of the casting. And so that’s really what we focused on in developing this alloy—improving its bend characteristics as much as improving its elongation in tensile conditions.”

Sizeable Challenge

One of the inherent challenges of producing very large parts lies in the question of: How do you manipulate these enormous pieces of sand, which weigh thousands of pounds, in order to build the sand core package? Similarly, handling large castings during heat treating is problematic. 

“You have to heat these things up to 1,000F and then plunge them into 8,000-gallons of quench media,” said Johnson. “But somehow you’ve got to keep them straight so they don’t distort. Those were the two principal challenges, handling the giant molds and components and then stopping the castings from distorting during heat treat.”

Once heat treated, the castings are blast cleaned; then, despite their size, TEI sends them through 100% X-ray because they are all safety-critical structural parts.

“We also laser scan every part and we target machine the datums,” said Johnson. “Then they go to a division of GM where they are fully machined. And after that, they go off to get a chemical conversion coating applied to passivate the aluminum and prepare it as a base for paint coatings.”

Reducing Capital Investment

While it may be true you have to spend money to make money, Moss understood from the start of the low-volume Celestiq program that he and his design lead peers from the chassis and interior groups were tasked with greatly reducing capital in their systems. Throughout the entire vehicle, the number of parts would be pared down from 300–400 to about 100. However, in the case of Moss’s body team, as they incorporated other groups’ systems into their complex single castings, their costs were naturally higher but enabled reduced cost overall for the vehicle. Moss challenged his team and colleagues in other areas to think differently and incorporate components and workstreams in ways they’d not previously considered. 

His six underbody castings also brought manufacturing costs down with so few parts to join. “We actually went one step further with these castings––we integrated a lot of general assembly attachment schemes into our designs to make their jobs simpler.”

Johnson agreed that fewer parts to assemble on the manufacturing floor contributed to major cost savings over the lengthy, conventional assembly of many stamped parts and fasteners. And indeed, shrinking the number of underbody components down to a mere six helped his customer realize significantly slimmer capital investment on the front end.  

Journey of Constant Discovery

Throughout the approximately 18-month design collaboration and prototyping process that started late in 2020, continuous close communication characterized the working relationship between Moss and Johnson. They often met or spoke by phone anywhere from three to 20 times a week—nearly three years later, Moss considers the folks at TEI his friends. He also emphasized how accommodating the TEI team was, even with some changes late in the process.  
“My joke was that I talked to Oliver more I talked to my wife during this program,” said Moss. “He is very accommodating­—we had many calls after hours. Anytime you do a clean sheet like this, it’s a challenge. I liken it to taking a road trip without a plotted course––I know I want to go somewhere but I don’t know how I’m going to get there. We had a few missed turns, and we discovered new things along the way. This was a journey of constant discovery.”
The Celestiq project was TEI’s first foray with Cadillac, but not with GM, having done prototypes for its EVs, tooling for the manufacturer and its foundries, as well as other specialized assignments including critical structural parts for GM’s racing Corvettes. 

“It was definitely their idea to build a complete vehicle underbody in just a small number of castings, which would be machined and would fit together very precisely. And it resulted in a very high-performance, very lightweight vehicle underbody,” said Johnson. “That was what they were striving to achieve. So, they came to us with that concept, and then our design engineers worked closely with them to first of all reduce even further the number of castings. We said, ‘well, where you’ve got three castings or four, we could make that one.  And they were surprised that we could actually get it down to so few parts. 
“We also provided them with a lot of rules for what thickness they could go down to, what kind of tolerances they needed to build in for these sand castings, and what kind of minimum radii they should be working to,” Johnson added. “We had lots of collaborative meetings with our designers working with theirs. And we convinced them about the benefits of using additive manufacturing for the internal cores.”

Looking back, Moss said the thing he’s most proud of concerning his part in the Celestiq program is really the whole process—just getting the rear rail casting designed, as well as its uniqueness.

“When you invent, there are a lot of challenges, and TEI was awesome to work with,” he said. “It means a lot to win this AFS award. We invented something, and so far, it looks like it’s going to be very successful. We didn’t know exactly where we were going to go … and it continued to morph into this entire underbody with six parts. It’s been quite a journey.”

Click here to view the column in the May 2023 Modern Casting digital edition.